Over recent decades, there has been a concerted effort to reduce atmospheric pollution caused by volatile solvents which are emitted during painting processes. Due to environmental concerns, volatile organic compounds (VOCs) have come under strict regulation by the government. Therefore, one of the major goals of the coating industry is to minimize the use of organic solvents by formulating waterborne coating compositions which provide a smooth, high gloss appearance, as well as good physical properties including resistance to acid rain. While the solvent-type coatings provide many benefits, such as that they are fast-drying, have a high hardness, a high abrasion-resistance, a high water-resistance, a high chemical-resistance and a low price, the waterborne coatings have environment-friendly benefits in that they are not flammable or explosive. The waterborne coatings use water as the system solvent and contain no poisonous chemicals. They require no or low amounts of volatile organic compounds.
The unique advantage of polyurethane dispersions (PUDs) in relation to surface coatings is their ability to form coherent film and to control the microphase morphology by controlling the relative amounts of soft and hard segments in polymer chain. These features allow PUDs to be employed in a wide variety of surface coating applications where mechanical properties are particularly crucial. High abrasion resistance, superior toughness, elastomeric properties, and high extensibility at low temperature are typical benefits. However, relatively high raw material cost in comparison with a typical acrylic emulsion has restricted its use in many industrial applications. To overcome this, polyurethane dispersions have been combined with other relatively inexpensive polymers to obtain a cost/performance balance because the properties of polyurethane (PU) and the polyacrylate (PA) complement each other. The composite materials of PU and PA are more outstanding in terms of adhesion, film-formability, non-stickiness, weather-resistance, elongation and strength of the film with excellent cost-performance balance. Accordingly, since the development of PU, the modification of the PU by the PA has been an active research topic in the art.
Two methods can be used to modify PU with PA: physical methods and chemical methods. The physical method is achieved by mechanical mixing. In the physical method, aqueous PA and PU dispersions (emulsions) are independently prepared first, and then both dispersions are mixed together under mechanical power. It is a very convenient method that makes it easy to control the composition of the final product. However, in such blends the superior performance properties may be compromised because of the incompatibility of the two. Such blended dispersion may suffer from instability.
For these reasons, the chemical modification technology currently plays a more important role. The chemical method is achieved by post-polymerization of acrylates. In the chemical method, the PU dispersion can be prepared first, and then acrylates and other vinyl monomers can be polymerized in the PU dispersion. In most cases, core-shell emulsion polymerization is adopted. PU particles are used as seed particles and the acrylates are polymerized within the PU particles due to high hydrophobicity of the acrylates. These hybrid dispersions are expected to provide the advantages of PA, such as excellent weather resistance, affinity to pigments as well as lower cost, and the advantages of PU, such as better mechanical stability, excellent adhesion, solvent and chemical resistance, and toughness.
U.S. Patent Application No.: 2009/0111934 A1 to Caideng Yuan discloses methods for the preparation of an aqueous PA modified PU dispersion, which includes three main steps: a) preparation of PA polymer or copolymer dispersion; b) preparation of PU prepolymer with carboxylic groups and neutralization treatment to the carboxylic groups; and c) dispersion and chain-extension of PU prepolymer by adding the PA dispersion into the PU prepolymer under vigorous agitation, or other mechanical operation. The result hybrid dispersion can be self-crosslinked by reaction between acetoacetoxy compound on PA particle and amine group on PU dispersion particles. A solvent, N-methyl-2-pyrrolidinone (NMP, b.p. 202-204° C.) was used as during the PU dispersion synthesis process. The use of NMP raises environmental concerns.
U.S. Publication No. 2004/0034146 discloses a complicated solvent free process for preparing hybrid PUA. The PU prepolymer was NCO free and there is no chain extension step in water. The viscosity of the PU prepolymer could be too high to be well dispersed into water. Additionally, dimethylol propane acid (DMPA) was used as an acid containing diol which provides water-dispersity of the PUA dispersion. However, DMPA is hard to dissolve completely, so that the final prepolymer may still contain particulate DMPA and the reaction was not complete.
The present inventors have solved the problem of inhomogeniety of the reaction system and have provided processes for preparing PUAs with superior mechanical performance, such as elongation, and tensile strength. The process is solvent free, smooth, and robust. PU prepolymer prepared according to the present invention has low viscosity, and contains no particulate DMPA.